The present invention relates to a magnetic tunnel junction device usable as a playback magnetic head of a high density magnetic disc device or a memory cell of a magnetic random access memory (MRAM) or an external magnetic field sensor.
A magnetic tunnel junction device has a tunnel magnetoresistance response as a function of applied magnetic field. Each magnetic tunnel junction in the device is formed of a ferromagnetic pinned layer, a ferromagnetic free layer, and an insulating tunnel barrier between and in contact with the two ferromagnetic layers. Magnetization direction of the ferromagnetic pinned layer is prevented from rotating, while magnetization of the ferromagnetic free layer is free to rotate between states parallel and antiparallel to the fixed magnetization of the ferromagnetic pinned layer. In the parallel state, the tunneling current is at a maximum and the tunneling resistance at a minimum. On the other hand, in the antiparallel state, the tunneling current is at a minimum and the tunneling resistance is at a maximum. The direction of the magnetization can be altered by an external magnetic field. Thus, the tunneling resistance is sensitive to magnetic field.
In xe2x80x9cMicrostructured Magnetic Tunnel Junctionsxe2x80x9d, Journal of Applied Physics 81 (8), Apr. 15, 1997, pp. 3741-3746, Gallagher, et al. reported on magnetic tunnel junctions using a tunneling barrier formed by plasma oxidizing an Al layer, which demonstrated large magnetoresistance (MR) ratios (15% to 22%) at room temperatures in low field. They reported on processes used to make magnetic tunnel junction devices with dimensions on the micron-to-submicron scale. For achieving two stable states in zero magnetic field, they employed an antiferromagnetic layer to exchange bias one of the electrode layers. This electrode layer is referred to as the pinned layer. The other electrode layer is referred to as the free layer.
FIG. 5 illustrates, a typical example of a magnetic tunnel junction having a tunneling barrier formed by oxidizing an Al layer, which provides a maximum MR ratio of 21%. In FIG. 5, the magnetic tunnel junction is on a substrate 54 of silicon (Si) and is formed of a series of layers of material stacked one on top of the other. The magnetic tunnel junction, in this example, comprises a bottom electrode 55 of platinum (Pt) (20 nm thick), an initial ferromagnetic layer 56 of a nickel-iron (Nixe2x80x94Fe) alloy (4 nm thick), an antiferromagnetic layer 57 of an iron-manganese (Fexe2x80x94Mn) alloy (10 nm), a fixed or ferromagnetic pinned layer 58 of Nixe2x80x94Fe (8 nm thick), a tunneling barrier layer 59 of aluminum oxide (Al2O3) formed by exposing a surface of an aluminum layer with 1.0 to 3.0 nm thick to an oxygen glow discharge, a ferromagnetic free layer 60 of cobalt (Co) (8 nm thick), and a top electrode of Pt (20 nm thick).
Magnetic tunnel devices including a tunneling barrier layer of Al2O3 formed by exposing a surface of an aluminum layer to the ambient atmosphere are described in JP-A 6-244477, JP-A 5-63254, JP-A 8-70148, JP-A 8-70149 and JP-A 8-316548. Which are laid-open publications of Japanese Patent Applications relating to inventions by T. Miyazaki and M. Etsumura.
In xe2x80x9cRelationship between the Barrier and Magnetoresistance Effect in Ferromagnetic Tunneling Junctionsxe2x80x9d, Journal of Japanese Applied Magnetism Society, Vol. 21, No. 4-2, 1977, pp. 493-496, N. Tesuka et al. reported on Fe/Al oxide/Fe junctions under varying oxidation conditions of the Al layer.
Fabrication of magnetic tunnel junctions at sizes below several microns is needed for their application to a playback magnetic head of a high-density magnetic disc device or a memory cell of a high density MRAM. In this case, the magnetic domain instabilities taking place in magnetic layers after a magnetic field has been applied cause a smaller signal to noise ratio. There remains a need, therefore, to fabricate magnetic tunnel junctions at sizes below several microns, which provide a larger signal to noise ratio in a magnetic field.
An object of the present invention is to accomplish the above-mentioned need.
A magnetic tunnel junction device according to one implementation of the present invention comprises a first pinning layer, a ferromagnetic free layer, a tunneling barrier layer, a ferromagnetic pinned layer, and a second pinning layer, which are stacked one on top of the other in this order. The first and second pinning layers may be in the form of antiferromagnetic layers, respectively. The ferromagnetic free layer is adjacent to the first pinning layer. Exchange coupling between the ferromagnetic free layer and the first pinning layer develops a magnetic anisotropy, which aligns magnetization of the ferromagnetic free layer along a track width direction. In other words, the first pinning layer has a pinning field, which pins a magnetization of the free layer in the track width direction. Exchange coupling between the ferromagnetic pinned layer and the second pinning layer develops a magnetic anisotropy, which aligns magnetization of the ferromagnetic pinned layer along a MR height direction. In other words, the second pinning layer has a pinning field, which pins a magnetization of the pinned layer In the MR height direction.
In the present application, a direction, in the plane of stalked layers of a magnetic tunnel junction, along the applied external magnetic field direction is called a MR height direction. A traverse direction, in another plane of the layers, forming right angles to the applied external magnetic field direction is called a track width direction.
A process of fabricating a magnetic tunnel junction comprises the step of forming a series of layers one on top of the others, the series of layers including a first antiferromagnetic pinning layer, a ferromagnetic free layer, a tunneling barrier layer, a ferromagnetic pinned layer, and a second antiferromagnetic pinning layer. The process also comprises the step of heating the layers at a temperature higher than a blocking temperature TB2 of the material of the second antiferromagnetic pinning layer in a magnetic field directed parallel to a MR height direction. The process further comprises the step of heating the layers at a temperature higher than a blocking temperature TB1 of the material of the first antiferromagnetic pinning layer in a magnetic field directed parallel to a track width direction.
It is required for suppressing noise upon sensing an external magnetic field to continuously vary the direction of magnetization in a ferromagnetic free layer of a magnetic tunnel junction after application of the field. For accomplishing this continuous variation, it is effective to develop a unidirectional magnetic domain, in the free layer, having a magnetic anisotropy, which aligns magnetization of the free layer along a track width direction that form right angles to the applied external magnetic field direction. There is a relation between noise and the magnetization direction of the free layer, which direction rotates upon application of external magnetic field. If the magnetization of the free layer is aligned in a MR height direction that is parallel to the applied external field direction, variation in the direction of magnetization due to the applied external field is in discontinuous magnetic domain displacement mode, thus providing a magnetoresistance (MR) curve with hysteresis. In the case where the magnetization of the free layer is aligned in the track width height direction, variation in the direction of magnetization due to the applied external field is in continuous magnetic domain rotation mode, thus providing a magnetoresistance (MR) curve without hysteresis.
According to one implementation of the present invention, therefore, the antiferromagnetic pinning layer is employed to exchange bias the free layer to induce a unidirectional magnetic anisotropy. The magnetic field induced due to the exchange coupling between the pinning layer and the free layer is larger in magnitude than the intrinsic magnetic anisotropy in the free layer. The magnetic anisotropy developed aligns magnetization in the free layer along the track width direction that forms right angles to the MR height direction. This configuration has demonstrated a large signal to noise ratio because, upon application of an external magnetic field along the MR direction, variation in direction of magnetization due to the applied field is in continuous magnetic domain rotation mode.
In order to rotate anisotropy in the free layer with anisotropy in the pinned layer unaltered for providing juxtaposed anisotropy relation, it is necessary to heat the free layer in magnetic field at a temperature lower than a temperature at which the pinned layer is heated by at least 50xc2x0 C. The free layer and the first pinned layer are fabricated to have a blocking temperature TB1 lower than the temperature for heating the first pinned layer by at least 50xc2x0 C. The temperature for heating the free layer is set at a temperature immediately above the blocking temperature TB1. Heating under this condition has proved to be effective in rotating anisotropy in the free layer to align magnetization along the track width direction while maintaining magnetization in the pinned layer, which has been aligned along the MR height direction.
The magnetic discs operate at temperatures around 100xc2x0 C. If the magnetic tunnel junction is used for a magnetic disc, it is desired to set the blocking temperature TB1 higher than 150xc2x0 C. (TB1 greater than 150xc2x0 C.) for its operation stability.
The magnetic field Hex1 developed due to magnetic coupling with the first antiferromagnetic pinning layer needs to be sufficiently larger than the intrinsic magnetic anisotropy induced in the free layer. Thus, it is desired to set Hex1 larger than 20 Oe (oersted). That is, Hex1 greater than 20 Oe.
Management of the magnitude of magnetic field developed in the free layer due to magnetic coupling Is important because the magnitude of this field determines sensitivity of an external magnetic field sensor. Locating an interface layer of nonmagnetic metal between the first pinning layer and the free layer makes this management. According to another implementation of the present invention, a magnetic tunnel Junction device comprises a first pinning layer, an interface layer, a ferromagnetic free layer, a tunneling barrier layer, a ferromagnetic pinned layer, and a second pinning layer, which are stacked one on top of the other in this order. The first and second pinning layers may be in the form of antiferromagnetic layers, respectively. The ferromagnetic free layer is adjacent to the first pinning layer with the interface layer interposed between them. Exchange coupling between the ferromagnetic free layer and the first pinning layer develops a magnetic anisotropy, which aligns magnetization of the ferromagnetic free layer along a track width direction. In other words, the first pinning layer has a pinning field, which pins a magnetization of the free layer in the track width direction. Exchange coupling between the ferromagnetic pinned layer and the second pinning layer develops a magnetic anisotropy, which aligns magnetization of the ferromagnetic pinned layer along a MR height direction. In other words, the second pinning layer has a pinning field, which pins a magnetization of the pinned layer in the MR height direction.
Fabricating the interface layer to have a thickness t falling in a range, i.e., 1 nmxe2x89xa6txe2x89xa610 nm, has proved to be effective in developing magnetic field in the free layer with appropriate magnitude for the external field magnetic sensor.
The first pinning layer may be in the form of a biasing ferromagnetic layer instead of the antiferromagnetic layer. In this case, the process step of aligning magnetization in the free layer in the track width direction requires a magnetic field that surpasses coercive force in the biasing ferromagnetic layer in heating the layers.